In response to the demand for more reliable and higher capacity data storage and retrieval systems, there is considerable activity in the research and development of optical disk recording systems. These systems utilize a highly focused modulated beam of light, such as a laser beam, which is directed onto a recording layer which is capable of absorbing a substantial amount of light. The heat thusly produced causes the light-absorbing material in the areas struck by the highly focused laser beam to change chemically and/or physically, thus producing a concomitant change in optical properties, e.g., transmissivity or reflectivity, in the affected area. For readout, the contrast between the amount of light transmitted or reflected from the unaffected parts of the absorbing layer and from the marked areas of the layer is measured. Examples of such recording systems are disclosed in numerous U.S. patents such as U.S. Pat. Nos. 3,314,073 and 3,474,457.
The simplest optical disk consists merely of a dimensionally stable solid substrate on which is coated a thin layer of light-absorptive material such as a metal layer. When the light-absorptive layer is struck by an intense beam of coherent light, such as from a laser source, the light-absorptive material is either vaporized and/or thermally degraded, thereby producing a very small marked area which exhibits different transmissivity or reflectivity than the adjacent unmarked layer. A more advanced laser recording medium is disclosed in Nam, U.S. Pat. No. 4,410,581 in which a single recording layer is completely encapsulated between an intermediate layer of solvent-resistant plastic material formed on a transparent substrate and a protective solvent-based plastic layer formed on the recording layer. In this instance, the encapsulated recording layer is imaged by a laser beam passing through the transparent substrate to burn a very small hole in the layer. In this context, the term "transparent" means that the material will transmit at least 85% of any light directed into it having a wavelength of 810-830 um.
It is obvious that when the information is recorded in a permanent manner as discussed above, it may not be erased and new information be written again on the disk. This limitation is vacated by using magneto-optical techniques to record, read, erase, and re-write information. These techniques are represented by a different class of optical disks, named magneto-optical disks.
A magneto-optical disk, as shown in FIG. 1, comprises a magnetic film 610C deposited on a transparent substrate 690. Magnetic layer 610C is overall magnetized as the vectors 691 indicate. The write-in or recording procedure is performed by applying a bias external magnetic field 693, which reverses the direction of the magnetized particles when a small area is heated by a focused beam 692 and their temperature is raised beyond the Curie Temperature. This arrangement allows not only writing information but also erasing the disk and re-writing. This type of optical disk can be erased by a constant flood overall exposure of the magnetic areas with a laser beam in the presence of a bias magnetic field.
The structure in actual practice is more complicated than the simplified view of FIG. 1. FIG. 2 illustrates a typical magneto-optical disk in more detail. It comprises a defocusing layer 710A which usually is made of polycarbonate material, a dielectric enhancement layer 710B, a magnetic layer 710C, a metal deflector layer 710D, a dielectric barrier layer 710E, and a protective layer 710F. A laminated adhesive bond layer 725 is used to attach this system onto a substrate 790. The dielectric layers may be nitrides or oxides that give optical enhancement properties as well as good barrier properties. The metal or reflector layers may be aluminum, titanium or chromium, and the like which give good optical reflection properties as well as barrier properties.
As mentioned above, a laser beam may be used in cooperation with a bias magnetic field selectively to reverse the magnetic vectors and write magnetic information on the disk. This operation requires a high intensity laser beam, while reading the information is conducted by the use of a lower intensity laser beam. The laser beam which is used for reading the information has to be polarized. The angle of polarization of the beam changes when the beam passes through a magnetic field. Depending on the polarization or the direction of the magnetic vectors containing the information as compared to the direction of the vectors in the background, the angle of polarization changes accordingly. This can be detected through a number of conventional mechanisms and can be converted to electrical signals which in turn may take another desired form of energy. Since the change in the angle of polarization is very small, usually of the order of 1 to 2 degrees, noise in the form of birefringence by external factors is a problem.
Conventional devices such as the one shown in FIG. 7 may be utilized to measure the birefringence of the system, preferably when there is no information on the disk for better evaluation. Very small birefringence noise may be detected through a system containing 45 degree prisms and lenses by means of differential amplifiers, having as output an electrical signal. When glass is used as the defocusing substrate or layer there is no problem of birefringence since the defocusing layer can have very high uniformity and flatness. However, when plastic materials are being used, such as polycarbonate, in order to make the system less expensive and affordable to the general public, birefringence becomes a major problem. This is due to localized strains in the disk which result from stresses during processing, especially when two single sided disks are joined to form a double sided disk.
As mentioned above, a magnet, preferably an electro magnet, imposes an external field on the magnetic recording layer. A focused laser provides local temperature increases as high as 500K. Magnetic reversal or switching of the magnetic vectors on the films occurs in areas heated above the switching temperature by the focused laser beam. The imposed magnetization persists when the film cools down to room temperature. This process is called thermo remanent magnetization. Thus, as aforementioned, the writing is conducted by an intense laser beam in the presence of a bias magnetic field, of approximately 300 Oersteds (Oe). Erasing may be forced by using a continuous laser and a bias magnetic field of approximately -300 Oe. Information retrieval, otherwise called readout, is conducted by probing local magnetization status with a focused laser beam, which is continuous and of rather low power on the order of approximately 2 mW. During information retrieval or readout, no external or bias magnetic field is applied. The film temperature may be as high as 360K without affecting the information on the film. Local magnetization status determines polarization of the reflected beam by a phenomenon called "reflective Kerr effect". A differential amplifier converts the polarization differences, which are sensed by this process, to a digital electric signal.
The desired properties of optical recording media are (1) high sensitivity, (2) high signal-to-noise ratio (SNR), which may be highly affected by birefringence due to stresses within the protective layer in the case of magneto-optical disks, (3) high tolerance to material variation, contaminants and other defects, and (4) high archival stability after extended storage and/or recording and readout (See Bartolini, J. Vac. Sci. Technology, Vol. 18, No. 1, Jan./Feb. 1981, p. 70.). Based upon these criteria, a considerable amount of research has been and continues to be carried out directed to obtaining the best possible disk materials.
There is an enormous and rapidly growing abundance of patent references regarding different aspects of optical disks. Examples of different magneto-optical systems are described in U.S. Pat. Nos. 3,224,333, 3,472,575, 4,670,316, 4,684,454, and 4,693,943, among others.
Among the plethora of patents describing double-sided disks comprising two single-sided disks bonded with an adhesive, are represented U.S. Pat. Nos. 4,760,012, and 4,571,124 and Japanese Patent Application Nos. 62/042347, 63/050932, and 63/137893.
To improve flatness, promote the creation of a rugged structure, and avoid corrosion of the active layers, especially in the case of magneto-optical disks on plastic substrates, it is highly preferable to use an appropriate adhesive to bond the two single-sided disks instead of leaving a gap between the respective surfaces of the disks containing the information.
The methods, which have been used so far, however, to bond the two magneto-optical single-sided disks on each other, produce high birefringence noise, which according to the instant invention may be reduced considerably, and yield products of highly improved quality. This is mainly achieved by applying pressure under vacuum only to those zones of the disks which are outside the information surfaces.
Japanese Patent Application 62/213749 discloses a method, in which the external peripheral edge portion of the adhesive surface of at least one substrate (single-sided disk) projects in the direction of the inner side (adhesive surface side) from the portion corresponding to the recording sector. Thus, when the two substrates are bonded together by adhesive, the pressure applied to this peripheral edge is higher during bonding. Use of this technique is intended to improve the aesthetics of the periphery of the disk. In accordance with the teachings of this application, the air bubbles mixed in with the adhesive are either squashed or forced out. As a result the cloudiness caused by the air bubbles in the periphery of the adhesive layer is prevented and transparency is improved.
U.S. Pat. No. 3,282,763 describes a method of adhering a light reflector unit to a mounting surface by using a flexible adhesive backing and performing the adhering step under vacuum. More particularly, it describes a method of adhering a light reflector unit having an inner cavity to a mounting surface, comprising the steps of: confining air pressure within the inner cavity by positioning a flexible adhesive backing in hermetically sealed relation across the opening to the inner cavity; positioning the reflector unit so that the exposed adhesive face of the flexible backing is adjacent the mounting surface; exhausting air pressure from about the reflector unit to cause confined air pressure in the inner cavity of the reflector unit to urge the central portion of the adhesive face of the flexible backing outwardly into continuous and intimate adhering contact with the mounting surface; and pressing the peripheral portion of the exposed adhesive face into adhering contact with the mounting surface while maintaining an area of decreased pressure about the reflector unit.
None of the above two references recognizes, suggests or implies any solution to the problem of birefringence noise in double-sided magneto-optical disks bonded with an adhesive.